As the scores are not avibalbe there, we'd need them here to investigate. Possibly MuseScore crashs at some point, like when exporting SVG or MP3 maybe and if so would do the same the desktop app as in the 3.0 backend on musescore.com

Wouldn't you think that the website would record "2018-12-17 03:03:16 MuseScore 3.4.5.6 crashed while rendering mp3 for https://m......" so somebody could look at it and tell us that that's what happened? Isn't that a better method than haruspicy, myomancy or pretending to be deaf?